Growth of piezoelectric bismuth oxide



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. 1 ,5 tinitcd i 3,470,100 GROWTH OF PIEZOELECTRIC BISMUTH OXIDE AlbertA. Ballman, Woodbridge, N.J., assignor to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York No Drawing.Filed Jan. 25, 1966, Ser. No. 522,840

Int. Cl. H01v 7/02; B01j 17/00; C04b 35/00 US. Cl. 252-629 1 ClaimABSTRACT OF THE DISCLOSURE This invention relates to a technique for thegrowth of single crystal bismuth trioxide (Bi203). More particularly,the present invention relates to a technique for the growth of opticallyactive bismuth trioxide manifesting high piezoelectric activity.

Thus far, it has been concluded that bismuth trioxide exists in fourcrystallographic modifications, namely, (a) a stable low temperaturemonoclinic form designated oc-Bi O (b) a metastable body-centered cubicform designated 'y-Bi O (c) a tetragonal form designated {i-Bi O and (d)a single cubic form. Although some interest has been generated in thesecompositions, the literature has heretofore been totally silent withregard to the electrical properties thereof, so negating its potentialin the electronics industry.

In accordance with the present invention, a technique is described forthe preparation of single crystal Bi O manifesting optical activityphotoconductivity and high piezoelectric activity, so suggesting its usein electrical optic devices, electromechanical transducers or inacoustic amplifiers. The inventive technique involves growing from ahigh purity melt comprising Bi O and small quantities of at least oneoxide selected from among the dioxides of germanium and silicon or theoxides of gallium, titanium and aluminum. The resultant single crystalcomposition is of good quality, evidences electromechanical couplingcoefficients in excess of 25 percent and is found to belong to the rarepoint group 23.

An important aspect of the present invention lies in the use of specificmelt compositions, for example, those containing critical proportions ofBi O and additive oxide, hereinafter designated MeO for convenience. Inthe growth of single crystal Bi O as described, it is essential that themol ratio of Bi O to MeO be within the range of 3:1 to 12:1. Thepreferred range is from 5:1 to 7:1, an optimum being found to correspondwith the approximate mol ratio of Bi O /MeO of 6: 1.

It has been determined that the use of mol ratios of Bi O /MeO less thanthe noted 3:1 minimum results in an increased incidence of bismuthcompounds other than the desired Bi O Similarly, studies on the growthof Bi O have revealed that the use of mol ratios in excess of the 12:1maximum result in the formation of the on form 0f Bi203.

In accordance with the inventive technique, it has been determined thatthe desirable physical and electrical properties discussed hereinabovecan successfully be generated in Bi O obtained by any prior artprocedure. However,

:ented Sept. 30,1969

it has been found essential to utilize high purity starting materials.Thus, it has been found that the Bi O source material must evidence aminimum purity of 99.9 percent. Similarly, the oxide materials employedin the practice of the present invention must necessarily evidence apurity of at least 99.9 percent. Studies have revealed that slightdeviations from these minimum impurity levels result in failure togenerate either piezoelectric or optical activity in the resultant Bi OWith regard to the oxidic materials found suitable in the practice ofthe present invention, it has been found that at least one compound fromamong germanium dioxide (GeO silicon dioxide (SiO gallium oxide (Ga Otitanium oxide (TiO and aluminum oxide (A1 0 is required to obtain thedesired properties in Bi O Examples of the application of the presentinvention are set forth below. They are intended merely as illustrationand it is to be appreciated that the processes described may be variedby one skilled in the art without departing from the spirit and scope ofthe invention.

The examples are in tabular form for convenience and brevity. Each setof data in the table is to be considered as a separate example sinceeach set of data was obtained in a separate process. The procedurefollowed in the examples is as follows:

A mixture of the starting materials, obtained from commercial sources,was weighed into a platinum crucible and heated to a temperature of theorder of 935 C., the melting point of the mixture. Heating was effectedby coupling the crucible with an RF induction heater. The crucible,

together with its contents was then permitted to attain a temperature of935 C. at which point the charge was entirely liquid. Next, a 30 ml.platinum wire was inserted into the melt. The Czochralski technique ofpulling crystals from the melt was then employed to grow Bi O toone-half inch diameter at a growth rate of one-half inch per hour. Inorder to obtain preferred orientation single crystal Bi O obtained inthis manner was subsequently employed in seed crystal form and Bi Ogrown upon the seed. The data set forth below is based upon Bi O grownupon seed crystals.

After cooling, the crystals were tested qualitatively for piezoelectricactivity by means of the well known Giebe- Scheibe test. Thereafter,optical activity was determined by passing light of a fixed frequency(white light) through a given thickness (1 mm.) of Bi O and bringing itto extinction. Following, rotation was effected to extinction again andthe rotation for a given thickness measured.

Photoconductivity was determined by connecting the output from thepiezoelectric detector (employed in the Giebe-Scheibe test) to anoscilloscope, resonant frequency of the Bi O crystal being evidenced bymultiple peaks. Following, the crystal was exposed to a white lightsource, so resulting in the loss of the peaks and indicating a change inconductivity of photoconductivity.

Finally, the electromechanical coupling coefficient of the Bi O wasmeasured, that is, the degree of efficiency of the piezoelectric body intransforming electrical energy to mechanical energy was determined. Inorder to determine the coupling coefficient a slice in disk form wastaken off a Bi O crystal normal to the growth direction and electrodedby applying silver paste to the major faces of the disk and firing byconventional techniques. Following, the electroded crystal was studiedby scanning the frequency spectrum in search of a strong resonancepoint. The coupling coefficient was determined by observing thefrequency shift from resonance to anti-resonance and computing it bymeans of well known formulae.

TABLE I Coupling B1 0 Purity, MeO Purity, CoeIIicient, Optical Photo-Example (grams) Percent (grams) Percent Product Giebe-Scheibe PercentActivity conductivity 2,795.66 99.9 Geog-104.59 v- 99.9 B1203 25 22lmn1.

2,795.66 99.9 Gu m-187.44 99.9 B1203 25 22,mm.

2,795.66 99.9 Bios-60.09 A. 99.9 131 0 25 22/mm.

+ :positive.

What is claimed is:

1. Single crystal, optical active piezoelectric composi- 1 ReferencesCited Kroger: Some Aspects of the Luminescence of Solids, 1948, page264.

Penkov et al.: Effect of Impurities on the Nuclear Quadrupole ResonanceSpectra of the aand 7-M0dlfic2ltion of Bi O Soviet physics, solid state,v01. 7, N0. 1,

July 1965, pages 145-147.

Gattow et al.: Ceramic Abstracts, page 2381', 1965.

15 =TOBIAS E. LEVOW, Primary Examiner R. D. EDMONDS, Assistant ExaminerUS. Cl. X.R. 252-501 @2 3 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent: No. 3, WO,lOO Dated September 30, 1969 Invent flAlbert A. Bellman It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Claim 1, column 3, line 10, change "optical" to --optically--.

SIGNED ANu SEALED DH; 2 3 19$ (SEAL) Atteat:

EdwardMFletcher, Jr.

WILLIAM E. SGHUYLER, JR. Attestmg Officer Oomissioner of Patents

